Stretch blow receptacle and molding method of the same

ABSTRACT

Improve the strength of the stretch blow receptacle and make the heat resistant treatment easier, by making the receptacle wall cross-section multi-layered in view of crystalline density, and the crystalline density of the surface layer high density. The inventions concerns a thin plastic receptacle  2  molded by stretch blowing of a bottomed preform  1  in the longitudinal and transversal directions. The wall cross-section is composed of inside and outside surface layers  2   a  whose crystalline density is high by stretching, a core layer  2   b  presenting a lower density than the surface layers  2   a , and an intermediate layer  2   c  presenting a graduated density between both layers  2   a,    2   b.

FIELD OF THE INVENTION

The present invention concerns a plastic stretch blow receptacle whosemain portions are molded thin, by stretching blowing of an injectionmolded bottom preform in the longitudinal and transversal directions.

BACKGROUND ART

Means for stretch molding of plastic receptacles for packaging such asbottles, wide-mouthed jars or the like include the cold parison methodfor stretch blowing a preformed bottomed preform into a receptacle byheat softening at a temperature higher than the glass transition point,and the hot parison method for immediately stretch blowing an injectionmolded soft and bottomed preform into a receptacle, while it maintains apotential heat higher than the glass transition point.

In the cold parison method, it is believed that the preform temperaturebecomes uniform and the crystalline density of the wall cross-section ofthe receptacle molded by stretch blowing is also uniformed, because apreform of ambient temperature is heated and softened by an externalheat, allowing to obtain a receptacle easy for heat resistant treatment.

In the hot parison method, it is believed that the preform temperaturetends to be uneven, because the stretch blowing should be completedwhile the preform is being softened by the potential heat, and thecrystalline density is lower that the cold parison method, making theheat resistant treatment deficient, without any remarkable difference instrength.

The present invention devised in consideration of the aforementionedsituation and has an object to provide a novel stretch blow receptacleand molding method wherein the wall cross-section is multi-layered inview of crystalline density, and the crystalline density of the surfacelayer is made extremely higher that the crystalline density of theinterior, thereby allowing to intend to increase the strength of thereceptacle and to make the heat resistant treatment easier.

DISCLOSURE OF THE INVENTION

The stretch blow receptacle of this invention according to theaforementioned object is a thin plastic receptacle molded by stretchblowing of a bottomed preform in the longitudinal direction andtransversal directions, wherein the wall cross-section of the receptacleis composed of inside and outside surface layers whose crystallinedensity is high by stretching, a core layer presenting a lower densitythan the surface layers, and an intermediate layer presenting agraduated density between both layers, and the area of the core layer atthe receptacle wall cross-section varies in proportion to theincrease/decrease of the thickness of the wall cross-section, and theintermediate layer intervenes with a graduated crystalline density,independently of the size of the area of the core layer.

Moreover, the stretch blow receptacle of this invention is the one madeof polyethylene terephthalate, the crystalline density of the surfacelayers is equal or superior to more or less than 1.4, the crystallinedensity of the core layer is lower than the surface layer as 1.32 to1.36, and the crystalline density of the intermediate layer interposedbetween both layers is graduated from about 1.4 to about 1.32.

Further, the molding method of this invention is the one comprisingsteps of quenching a bottomed preform to or less than the orientationtemperature during the injection molding; forming the wall inside andoutside surfaces of the preform into a half set cover layer, in relationto the cooling temperature and time; removing the preform from the diemaintaining the configuration by the half set cover layer; andthereafter stretch blow molding while the temperature at the middle ofthe preform is in the amorphous temperature range or in the proximitythereof, the outside surface temperature of the preform is elevating,and the half set cover layer maintains a sufficient tensile resistance,for molding into a receptacle of multi-layered structure having a wallcross-section composed of inside and outside surface layers whosecrystalline density is high in respect of the crystalline density, acore layer presenting a lower density than the surface layers, and anintermediate layer interposed with a gradient crystalline densitybetween both layers.

Moreover, it is the aforementioned molding method, wherein the preformis made of polyethylene terephthalate, the outside surface temperatureof the preform is elevating and 95° to 115° C., the temperature at thewall middle is in a range of 125° to 140° C., and the preform is moldedby stretch blowing, while a temperature difference of more or less than30° C. is maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 an illustrative drawing showing a configuration and a partialcross-section of the stretch blow receptacle according to the presentinvention and a preform by a longitudinal cross-section;

FIG. 2 a relation diagram of thickness and crystalline density atrespective measurement area of the same stretch blow receptacle;

FIG. 3 a relation diagram of thickness and crystalline density of areceptacle manufacture by stretch blowing a preform per 10 sec after theremoval from the die; and

FIG. 4 a relation diagram of thickness and crystalline density of areceptacle manufacture by stretch blowing a preform per 15 sec after theremoval from the die and distribution.

BEST MODE FOR CARRYING OUT THE INVENTION

In the drawing, 1 is a bottomed preform made of a thermoplasticmaterial, for example polyethylene terephthalate, composed of a mouth11, a body 12 extending to the same and a bottom 13, and molded byinjection filling a die with molten plastic material.

A wide-mouthed receptacle 2 comprises a mouth 21 by the mouth 11 of thepreform 1, a body 22 and a bottom 23 thinly stretch blown from the underside of the mouth, while the wall cross-section is composed of insideand outside surface layers 2 a whose crystalline density is high, a corelayer 2 b presenting a lower density than the surface layers 2 a, and anintermediate layer 2 c presenting a graduated density between bothlayers 2 a, 2 b.

The wide-mouth receptacle 2 made of multi-layered wall cross-section inrespect of the crystalline density can be manufactured by the stretchblow formation according to the hot parison method. However, it isnecessary to form the inside and outside surface layers of the preform 1into the half set cover layers 1 a, 1 b in relation to the coolingtemperature and time by quenching the preform 1 at the temperature notmore than an orientation temperature during the injection molding, inorder to make the crystallite density of the inside and outside surfacelayers 2 a high and that of the core layer 2 b low.

This half set cover layer 1 a is made of a skin layer generated by thecontact of molten plastic with the cavity face, when the preform 1 isinjection molded, and the thickness of the skin layer can be controlledin relation to the cooling temperature and time. The molten plasticflows over this skin layer and fills the cavity, and the skin layer ispulled by the flowing molten plastic increasing the thickness, therecreating a flow orientation. It is supposed that this flow orientationwould not be lost so long as the skin layer forms the surface of thepreform 1 as half set cover layer 1 a, and the crystallization degree ofthe surface layer 2 a increases by the combination of this floworientation with the orientation by stretch.

When the molding material is polyethylene terephthalate, it ispreferable to set the cooling temperature of the die during theinjection molding of the preform 1 to more or less than 15° C. Inaddition, it is necessary to limit the cooling within a period of timeallowing to form the inside and outside surfaces of the preform 1 withthe half set cover layers 1 a, 1 a having a temperature equal orsuperior to the glass transition point, and the configuration of thepreform 1 can be maintained sufficiently by the half set cover layer 1a.

This cooling time is different according to the thickness of the body 11to be blown, and the cooling time (for instance 6 to 10 sec) is set inproportion to the thickness as tendency. If the cooling time is short incomparison with the thickness, the surface half set cover layer 1 a isgenerated thinly, and softens extremely by the potential heat within theperiod of time leading to the stretch blowing, making impossible toobtain a sufficient tensile resistance of the preform surface layer bythe half set cover layer 1 a and, on the contrary, if it is too long,the half set cover layer 1 a is generated thickly more than necessary,causing problems of stretch blowing.

However, according to the configuration of the receptacle to be formed,even if the thickness of the preform is identical, a different coolingtime may be set in order to preserve inside a potential heat adapted tothe molding of the receptacle.

The injection molded preform 1 is removed from the die holding the mouth11 by a well-known means, transferred immediately to the blow die in ahollow state as it is and set in the blow cavity by die compression.Then, as usual, the mouth 11 is closed by a blow core and, thereafter,the stretch blow is performed in the longitudinal direction by a middlestretch rod, and in the transversal direction by blow air, to mold intothe aforementioned wide-mouth receptacle 2 wherein the mouth 11 of thepreform 1 has become the mouth 21 as it is, and the body 22 is thin.

In order to obtain a wide-mouth receptacle 2 of the wall cross-sectionhaving an intermediate layer 2 c, 2 c presenting a gradient densitybetween the inside and the outside surface layers 2 a, 2 a whosecrystalline density is high and, a core layer presenting a lower densitythan the surface layers, by this elongation blow, it is necessary tomold by stretch blow while the temperature at the middle of the preform1 is in the amorphous temperature range or in the proximity thereof, theoutside surface temperature of the preform 1 is elevating, and at leastthe outside half set cover layer maintains a sufficient tensileresistance.

When the preform 1 is made of polyethylene terephthalate, it ispreferable to mold the preform by stretch blowing while the outsidesurface temperature of the preform is elevating and 95° to 115 C.°, thetemperature at the wall middle is in a range of 125° to 140□, and atemperature difference of more or less than 30□ is maintained there.When the surface temperature is lower than 95□, and the temperature ofthe wall middle is in the range of such high temperature, the differenceof crystalline density between the surface layers 2 a and the core layer2 b becomes considerable, making difficult to generate an intermediatelayer 2 c linking both. As the result, when the body stretched thinly ispressed strongly, the outer surface layer often peels off, lowering alsothe buckling strength.

Moreover, when the temperature difference between the wall middle andthe surface temperature becomes less that 25° C. and the temperaturegradient becomes low by the radiation of inner heat along with the timeelapsed after the removal from the die, the difference in crystallinedensity between the surface layers 2 a and the core layer 2 b alsolowers, the density gradient between both of them, and the crystallinedensity also becomes averaged.

For the preform 1 of 4.4 mm in thickness formed under the moldingcondition of the embodiment mentioned below, the half set cover layer 1a softens unexpectedly at a high speed by the potential heat after theremoval from the die. FIG. 2 shows the measurement (by Ramanspectroscopy) results of the thickness and the crystalline density of awide-mouth receptacle 2 molded by stretch blowing 6 sec after theremoval from the die, at respective areas of panel A, shoulder B, body Cand bottom D of the wide-mouth receptacle 2 shown in FIG. 1.

These 6 sec after the removal from the die could be called the leavetime in case of the preform of 4.4 mm in thickness, and this leave timecan be set in its own way, if the thickness of the wall thicknessexerting a considerable influence on the potential heat changes, and italso means that the leave time can be set by controlling, through thecooling time, the degree of development of the half set cover layer 1 agenerated by cooling during the preform molding except for the wallthickness.

As for the wall thickness of the receptacle 2, it is 1.25 mm for thepanel A where the transversal stretch rate is minimum, 0.4 mm for thebottom B where the transversal stretch rate is maximum, 0.45 mm for theshoulder B, 0.5 mm for body C, resulting in 0.65 mm for the averagethickness.

Moreover, from respective curbs of crystalline density of FIG. 2, it canbe understood that all portions of the surface layer 2 a where thetemperature is elevating are within the thickness range of about 0.2 mm,and the intermediate layer 2 c presenting a gradient crystalline densityintervenes with a thickness range of about 0.2 mm, and the middleportion excluding them forms the core layer 2 c.

In any measurement area of the wide-mouth receptacle 2, the crystallinedensity high for the surface layer 2 a as equal or superior to 1.4 andeven the core layer 2 b supposed to be in amorphous state present ancrystallization of equal or superior to 1.32 in crystalline density and,from the comparison of respective measurement areas, the core layer 2 bpresenting a density lower than the surface layer 2 a varies inproportion to the increase/decrease of the thickness of the wallcross-section; however, the aforementioned intermediate layer 2 cintervenes with a gradient crystalline density within a certain rangeindependently of the size of the core layer 2 b.

The foregoing allows understand that even a receptacle whose globalthickness of the receptacle is molded by stretch blow similarly to anythickness of respective area composing a part of the receptacle 2,becomes a receptacle of multi-layered structure by a crystalline densitydistribution similar to the respective area.

FIG. 3 shows, as a comparative example, the crystalline density anddistribution of a receptacle 2 obtained by stretch blowing of thepreform 1 about 10 sec after the removal from the die and, according tothis, the crystalline density of respective measurement area appearsaveraged globally in a range 1.36 to 1.38, even if there is some up anddown differences of a certain degree.

FIG. 4 also shows, as a comparative example, the crystalline density anddistribution of a receptacle 2 obtained by stretch blowing of thepreform about 15 sec after the removal from the die and, according tothis, it can be understood that the crystalline density varies hardly inthe stretch blow after the time when 5 more sec have elapsed, and thecrystalline density of respective measurement areas are even moreaveraged.

It is believed that this is due to a global averaging of crystallizationby stretching, as the result of lost of density difference between bothsurface layers 2 a and the core layer 2 b due to an even generation oftensile resistance during the stretch blow all over the preform, by thefact that the temperature of the preform surface layer raises by thepotential heat along with the lapse of time, provoking softening, andthe preform temperature is balanced as the whole, on the contrary, dueto the decrease of inner temperature by radiation.

This crystalline density can be the to be a crystalline density anddistribution almost equivalent to the crystalline density of areceptacle molded by stretch blowing according to the cold parisonmethod; however, material deterioration or distortion can be generatedeasily, as the crystallization by a gradual cooling develops during thestretch blowing with this period of time.

Consequently, in the stretch blowing after 10 sec or more after theremoval from the die, the flow orientation disappears by the softeningof the half set cover layer 1 a by the potential heat, and thecrystallization degree by stretching reduces as the tensile resistancelowers, averaging as the whole the crystalline density of the surfacelayer 2 a and the core layers 2 b in the wide-mouth receptacle 2, andthere is no more room for generating an intermediate layer 2 c ofgraduated crystalline density between both layers.

In FIG. 2, the crystalline density of 1.4 or more of the surface layer 2a can be the extremely high density for a receptacle made ofpolyethylene terephthalate and becomes per se one having a heatresistance in a certain sense. However, molding distortion due to aforced stretch remains largely in such a surface layer 2 a.

On the contrary, the core layer 2 b of low density is formed bystretching a preform middle portion presenting a highest potential heat,being in the amorphous state, and presenting an extremely low tensileresistance, according to the stretch of the half set cover layer 1 a tobe the surface layer 2 a of the wide-mouth receptacle 2, and is in astate where the molding distortion due to the stretch is almost absent,because the crystallization is generated by the temperature decrease bythe process of this stretch.

In the receptacle having a wall cross-section of multi-layered structurewherein the intermediate layer 2 c with a density gradient is interposedbetween the surface layer 2 a and the core layer 2 b presentingdifferent crystalline densities, the fragility of the surface layer 2 aagainst the impact force by the molding distortion is reinforced by aninner dumping of the core layers 2 b subjected to no or little moldingdistortion and its drop strength becomes excellent, due to a stillcloser linkage of the both by the intervention of the intermediate layer2 c.

The heat resistance can be improved by relaxing the molding distortionof the surface layer 2 a by heat treatment. This relaxation treatmentcan be performed by applying the blow pressure as it is for around 15sec, without decompressing immediately after the elongation blowing, asperformed usually, in a blow die set at a temperature of more or lessthan 116 C.°, pressure welding the molded receptacle 2 to the cavityface, and heating.

For the wall cross-section, in a structure wherein the crystallinedensity changes extremely suddenly from high density to low density,that area of sudden change in crystalline density tens to peel offeasily; however, a wall cross-section wherein the transition from thehigh density of the surface layer 2 a to the low density of core layergenerates through the intermediate layer 2 c having a graduatedcrystalline density peels off hardly even if it is in a multi-layeredstate in respect of the crystalline density. In view of the forgoing,the stretch blow receptacle according to the present invention can bethe to be further improved in the strength that the conventional one,and also improved in heat resistance.

Certainly, the aforementioned embodiment concerns a wide-mouthreceptacle 2, but is goes without saying that it can be applied to anarrow-mouth bottle and, therefore, the present invention is notrestricted to the wide-mouth receptacle. The molding material is alsonot limited to polyethylene terephthalate, and it can also be applied tothe other thermoplastic materials adopted as molding material ofpackaging receptacles.

Embodiments (1) Preform Molding material polyethylene terephthalateDimensions Mouth diameter Ø59 mm Body diameter (at the middle) Ø56 mmLength of elongated par 63 mm Wall thickness 4.4 mm (2) Moldingcondition (injection molding) Injection mold temperature (chiller)   15°C. Resin Temperature  254° C. Injection time 12.0 sec Cooling time 7.0sec (3) Molding condition (stretch blow) After removal from die 6.0 secDie temperature  116° C. Preform surface temperature 96.2° C. Blowpressure 24 Kgf/cm2 Blow time 15.0 sec Elongation rate (longitudinal)2.53 times (transversal) 1.95 times (4) Mold product (wide-mouthreceptacle) Dimensions Mouth diameter same as perform Body diameter Ø95mm Average thickness (except for 0.58 mm mouth and bottom face portion)Average density 1.348 mm Thickness and crystalline density (refer toFIG. 2) distribution Measurement method and instruments Densityorientation degree measurement Raman spectroscopy JEOL: MagnetiscopicRaman Spectrum JRS-SYSTEM2000 Temperature measurement Ultravioletthermograph TVS-2000 Mk2 made by Nippon Aobionics Co., Ltd. Measuringmethod of median temperature Measure the wall cut section by thermographUsed molding machine SB3-100LL-20 made by Aokiko Institute Ltd.

INDUSTRIAL APPLICABILITY

In the receptacle having a wall cross-section of the multi-layeredstructure wherein the intermediate layer with a density gradient isinterposed between the face layer and the core layer presentingdifferent crystalline densities, the fragility of the surface layeragainst the impact force by the molding distortion is reinforced by aninner dumping of the core layer subjected to non or little moldingdistortion and its drop strength becomes excellent, due to a stillcloser linkage of the both by the intervention of the intermediatelayer.

The heat resistance can be improved by relaxing the molding distortionof the surface layer by heat treatment. This relazation treatment can beperformed by applying the blow pressure as it is for around 15 sec,without decompressing immediately after the strech blowing, as performedusually, in a blow die set at a temperature of more or less than 116°C., pressure welding the molded receptacle to the cavity face andheating.

1. A thin plastic receptacle molded by stretch blowing of a bottomedpreform in the longitudinal direction and transversal direction,wherein: the wall cross-section of the receptacle is composed of insideand outside surface layers whose crystalline density is about 1.4 orgreater, a core layer having a lower crystalline density than thesurface layers, wherein the crystalline density of the core layer isfrom about 1.32 to about 1.36 and an intermediate layer having agraduated crystalline density crystalline between both layers.
 2. Thethin plastic receptacle of claim 1, wherein: the area of said core layerat a receptacle wall cross-section varies in proportion to anincrease/decrease of the thickness of the wall cross-section, and saidintermediate layer intervenes with a graduated crystalline density,independently of the size of the area of the core layer.
 3. The thinplastic receptacle of claim 1, wherein: said plastic receptacle is madeof polyethylene terephthalate, the crystalline density of said surfacelayers is about 1.4 or greater, the crystalline density of said corelayer is lower than the surface layer and is about 1.32 to about 1.36,and the crystalline density of the intermediate layer interposed betweenboth layers is graduated from about 1.4 or greater to about 1.32 toabout 1.36 in the range of from about 1.4 to 1.32 to 1.36.
 4. A moldingmethod of stretch blow receptacle comprising steps of quenching abottomed preform equal to or less than the orientation temperatureduring the injection molding; forming the wall inside and outsidesurfaces of the preform into a half set cover layer, in relation to acooling temperature and time; removing the preform from the die whilemaintaining the configuration by the half set cover layer; andthereafter stretch blow molding while the temperature at the wall middleof the preform is in an amorphous temperature range or in the proximitythereof, the outside surface temperature of the preform is elevating,and the half set cover layer maintains a sufficient tensile resistance,for molding into a receptacle of multi-layered structure having a wallcross-section composed of inside and outside surface layers whosecrystalline density is about 1.4 or greater, a core layer having a lowercrystalline density than the surface layers, wherein the crystallinedensity of the core layer is from about 1.32 to about 1.36 and anintermediate layer interposed with a gradient crystalline densitybetween both layers.
 5. The molding method of claim 4, wherein: saidpreform is made of polyethylenete rephthalate, the outside surfacetemperature of the preform is elevating and 95° to 115° C., thetemperature at the wall middle is in a range of 125° to 140° C., and thepreform is molded by stretch blow, while a temperature difference ofmore or less than 30° C. is maintained.